为该MySQL查询找到最佳索引

Question

I'm struggling to understand if I've indexed this query properly, it's somewhat slow and I feel it could use optimization. MySQL 5.1.70

  select snaps.id, snaps.userid, snaps.ins_time, usr.gender  
    from usersnaps as snaps  
    join user as usr on usr.id = snaps.userid  
    left join user_convert as conv on snaps.userid = conv.userid  
    where (conv.level is null or conv.level = 4) and snaps.active = 'N' 
and (usr.status = "unfilled" or usr.status = "unapproved") and usr.active = 1 
        order by snaps.ins_time asc

usersnaps table (irrelevant deta removed, size about 250k records) :

CREATE TABLE IF NOT EXISTS `usersnaps` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `userid` int(11) unsigned NOT NULL DEFAULT '0',
  `picture` varchar(250) NOT NULL,
  `active` enum('N','Y') NOT NULL DEFAULT 'N',
  `ins_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`,`userid`),
  KEY `userid` (`userid`,`active`),
  KEY `ins_time` (`ins_time`),
  KEY `active` (`active`)
) ENGINE=InnoDB;

user table (irrelevant deta removed, size about 300k records) :

 CREATE TABLE IF NOT EXISTS `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `active` tinyint(1) NOT NULL DEFAULT '1',
  `status` enum('15','active','approval','suspended','unapproved','unfilled','rejected','suspended_auto','incomplete') NOT NULL DEFAULT 'approval',
  PRIMARY KEY (`id`),
  KEY `status` (`status`,`active`)
  ) ENGINE=InnoDB;

user_convert table (size about : 60k records) :

CREATE TABLE IF NOT EXISTS `user_convert` (
  `userid` int(10) unsigned NOT NULL,
  `level` tinyint(4) NOT NULL,
  UNIQUE KEY `userid` (`userid`),
  KEY `level` (`level`)
) ENGINE=InnoDB;

Explain extended returns :

id  select_type table   type    possible_keys               key     key_len ref             rows    filtered    Extra
1   SIMPLE      snaps   ref     userid,default_pic,active   active  1       const           65248   100.00      Using where; Using filesort
1   SIMPLE      usr     eq_ref  PRIMARY,active,status       PRIMARY 4       snaps.userid    1       100.00      Using where
1   SIMPLE      conv    eq_ref  userid                      userid  4s      snaps.userid    1       100.00      Using where

Answer 1

I would recommend changing the userid index (assuming you're not using it right now) to have active first and userid later.

That should make it more useful for this query.

Answer 2

Using filesort is probably your performance killer.

You need the records from usersnaps where active = 'N' and you need them sorted by ins_time.

ALTER TABLE usersnaps ADD KEY active_ins_time (active,ins_time);

Indexes are stored in sorted order, and read in sorted order... so if the optimizer chooses that index, it will go for the records with active = 'N' and -- hey, look at that -- they're already sorted by ins_time -- because of that index. So as it reads the rows referenced by the index, the result-set internally is already in the order you want it to ORDER BY , and the optimizer should realize this... no filesort required.